1. Field of the Invention
The present invention relates to a current perpendicular to the planes (CPP) sensor with an in-stack biased free layer wherein the CPP sensor is either a CPP spin valve sensor or a tunnel junction sensor.
2. Description of the Related Art
The heart of a computer is typically a magnetic disk drive which includes a rotating magnetic disk, a slider that has write and read heads, a suspension arm above the rotating disk and an actuator arm. The suspension arm biases the slider into contact with a parking ramp or the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the actuator arm swings the suspension arm to place the write and read heads over selected circular tracks on the rotating disk where field signals are written and read by the write and read heads. The write and read heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a current perpendicular to the planes (CPP) sensor for sensing the magnetic field signals from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive or electrically nonconductive material spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer typically interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. The sensor is located between ferromagnetic first and second shield layers. First and second leads are connected to a bottom and a top respectively of the sensor for conducting a current perpendicular to the major thin film planes (CPP) of the sensor. This is in contrast to a CIP sensor where the current is conducted in plane parallel to the major thin film planes (CIP) of the sensor. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic field signals from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is parallel to the ABS, is when the current is conducted through the sensor without magnetic field signals from the rotating magnetic disk.
When the aforementioned material spacer layer is nonmagnetic and electrically conductive, such as copper, the current is referred to as a sense current, but when the material spacer layer is nonmagnetic and electrically nonconductive, such as aluminum oxide, the current is referred to as a tunneling current. Hereinafter, the current is referred to as a perpendicular current (Ip) which can be either a sense current or a tunneling current.
When the magnetic moments of the pinned and free layers are parallel with respect to one another the resistance of the sensor to the perpendicular current (Ip) is at a minimum and when their magnetic moments are antiparallel the resistance of the sensor to the perpendicular current (Ip) is at a maximum. Changes in resistance of the sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When the perpendicular current (Ip) is conducted through the sensor, resistance changes, due to field signals from the rotating magnetic disk, cause potential changes that are detected and processed as playback signals. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the sensor at minimum resistance.
Sensors are classified as a bottom sensor or a top sensor depending upon whether the pinned layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Sensors are further classified as simple pinned or antiparallel (AP) pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic AP layers that are separated by a coupling layer with magnetic moments of the ferromagnetic AP layers being antiparallel. Sensors are still further classified as single or dual wherein a single sensor employs only one pinned layer and a dual sensor employs two pinned layers with the free layer structure located therebetween.
The first and second shield layers may engage the bottom and the top respectively of the CPP sensor so that the first and second shield layers serve as the aforementioned leads for conducting the perpendicular current through the sensor perpendicular to the major planes of the layers of the sensor. The read gap is the length of the sensor between the first and second shield layers. It should be understood that the thinner the gap length the higher the linear read bit density of the read head. This means that more bits can be read per inch along the track of a rotating magnetic disk which enables an increase in the storage capacity of the magnetic disk drive.
Assuming that the aforementioned pinning layers are platinum manganese (PtMn) each pinning layer has a thickness of approximately 150 xc3x85 which increases the read gap in a dual CPP sensor by 300 xc3x85. This seriously impacts the linear read bit density of the read head. Further, the pinning layers significantly increase the resistance R of the sensor to the perpendicular current (Ip). The result is that the magnetoresistive coefficient dr/R of the sensor is decreased. The pinning layers also require extra steps in their fabrication and a setting process. After forming the sensor, the sensor is subjected to a temperature at or near a blocking temperature of the material of the pinning layer in the presence of a field which is oriented perpendicular to the ABS for the purpose of resetting the orientation of the magnetic spins of the pinning layer. The elevated temperature frees the magnetic spins of the pinning layer so that they align perpendicular to the ABS. This also aligns the magnetic moment of the pinned layer structure perpendicular to the ABS. When the read head is cooled to ambient temperature the magnetic spins of the pinning layer are fixed in the direction perpendicular to the ABS which pins the magnetic moment of the pinned layer structure perpendicular to the ABS. After resetting the pinning layer it is important that subsequent elevated temperatures and extraneous magnetic fields not disturb the setting of the pinning layer.
It is also important that the free layer be longitudinally biased parallel to the ABS and parallel to the major planes of the thin film layers of the sensor in order to magnetically stabilize the free layer. This is typically accomplished by first and second hard bias magnetic layers which abut first and second side surfaces of the spin valve sensor. Unfortunately, the magnetic field through the free layer between the first and second side surfaces is not uniform since a portion of the magnetization is lost in a central region of the free layer to the shield layers. This is especially troublesome when the track width of the sensor is sub-micron. End portions of the free layer abutting the hard bias layers are over-biased and become very stiff in their response to field signals from the rotating magnetic disk. The stiffened end portions can take up a large portion of the total length of a sub-micron sensor and can significantly reduce the amplitude of the sensor. It should be understood that a narrow track width is important for promoting the track width density of the read head. The more narrow the track width the greater the number of tracks that can be read per linear inch along a radius of the rotating magnetic disk. This further enables an increase in the magnetic storage capacity of the disk drive.
There is a strong-felt need to increase the magnetic storage of the disk drive as well as increasing the magnetoresistive coefficient dr/R of the CPP sensor.
An aspect of the invention is to provide an in-stack biasing structure, which is located within the track width of a current perpendicular to the planes (CPP) sensor, for longitudinally biasing the free layer of the sensor in a direction parallel to the ABS and parallel to the major planes of the layers of the sensor. In a preferred embodiment the biasing structure includes an iron pinned layer and a chromium spacer layer which is located between and interfaces the pinned layer and the free layer so that the pinned and free layers are magnetostatically coupled. The biasing layer structure further includes an antiferromagnetic (AFM) pinning layer which is exchange coupled to the pinned layer for pinning a magnetic moment of the pinned layer parallel to the ABS and parallel to the major planes of the layers of the sensor. The chromium spacer layer weakly antiparallel couples the iron pinned layer and the free layer and is in a direction to support the magnetostatic coupling between the pinned layer and the free layer. It has been found that a chromium layer with a thickness of approximately 10 xc3x85 weakly antiparallel couples the pinned and free layers. Because of the magnetostatic and AP couplings between the pinned and free layers the free layer is uniformly biased from a first side surface to a second side surface. This biasing is more uniform than the aforementioned first and second hard bias layers adjacent the side surfaces of the free layer since the hard bias layers result in overbiasing end regions of the free layer and restricting the employment of narrow track width sensors.
An aspect of the invention is that the free layer includes first and second free films with the first free film interfacing the material spacer layer and the second free film interfacing the chromium spacer layer. In a preferred embodiment the first free film is composed of only iron (Fe) and the second free film is composed of nickel iron (NiFe). Accordingly, the second free film is magnetically softer than the first film and imparts softness to the first film so that both the first and second free films rotate responsively to a field signal from a moving magnetic medium. Most importantly, however, is that the iron composition of the first film and the pinned layer in combination with the chromium spacer layer significantly increases the magnetoresistance dr/R of the CPP sensor. This is also true when only the pinned layer is composed of iron, but the dr/R is still further increased when the first free film is also composed of iron.
This embodiment of the invention may employ an AP pinned layer structure with a second AFM pinning layer pinning one of the AP pinned layers of the AP pinned layer structure or a self-pinning antiparallel (AP) pinned layer structure without an AFM pinning layer pinning the AP pinned layer structure. The self-pinning is accomplished by uniaxial anisotropies of the AP pinned layers which are oriented perpendicular to the ABS and, in combination, self-pin the magnetic moments of the first and second AP pinned layers perpendicular to the ABS and antiparallel with respect to each other. The use of the self-pinning scheme permits the employment of a single antiferromagnetic material, which material is used for the AFM pinning layer in the biasing structure. This is made possible by the fact that the AP pinned layer structure is self-biasing and does not require the AFM pinning layer. Accordingly, after fabricating the read head the magnetic spins of the AFM pinning layer in the biasing structure can be set by elevating the temperature at or near the blocking temperature of the AFM material in the presence of a field that is oriented parallel to the ABS and parallel to the major planes of the layers of the sensor. Upon removing the elevated temperature, the magnetic spins of the AFM pinning layer are set to pin the magnetic moment of the pinned layer parallel to the ABS and parallel to the planes of the layers of the sensor. This does not affect the perpendicular orientation of the AP pinned layers of the AP pinned layer structure since these layers are not pinned by an AFM pinning layer. The preferred AFM material for the pinning layer of the biasing structure is platinum manganese. In either the AFM pinned AP pinned layer structure or the self-pinned AP pinned layer structure it is preferred that the second AP pinned layer interfacing the spacer layer be composed of iron (Fe) and the first AP pinned layer be cobalt iron (CoFe). By making the second AP pinned layer of iron this further increases the magnetoresistive coefficient dr/R of the CPP sensor, as discussed hereinabove. Commonly assigned U.S. Pat. No. 6,127,053 is incorporated in its entirety regarding self-pinned AP pinned layer structures.
An object is to provide a CPP sensor wherein a free layer is more uniformly biased and a magnetoresistive coefficient dr/R is increased.
Another object is to provide layers of the sensor with compositions which increase the magnetoresistive coefficient dr/R of the sensor.
A further object is to provide a method for making the aforementioned CPP sensor.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.